(1) Field of the Invention
This invention relates to acoustic projectors for sonar use and more particularly to a lightweight acoustic projector that can be used by itself or in an array.
(2) Description of the Prior Art
Low frequency transducers having resonances below about 10 kHz have numerous applications, one of which is as a low frequency sonar projector. This acoustic wavelength corresponding to these frequencies is on the order of the size of naval mines, and thus can hunt for and/or classify them, as well as objects of similar size. Also, wavelengths of this size permit sonar location of buried objects, a task of interest to a wide range of commercial and governmental concerns. Unfortunately, current underwater projectors at these frequencies are large and heavy, and are cumbersome to use on many underwater vehicles.
The U.S. Navy is particularly interested in detecting objects in littoral environments for which small, unmanned submersible vehicles are best-suited. Because of the size constraint of the vehicles, it is necessary to keep the dimensions of the associated acoustic projector systems small, particularly along the protrusion dimensions. Acoustically, the desire is for an acoustic source level greater than 180 dB, while geometrically the projector systems need to be thin (less than 60-65 mm) for installation onto the sides of underwater vehicles and tow sleds ranging in diameter from 15 cm to over 2.4 meters. Conventional transducer designs used to generate high power sound waves at frequencies under 30 kHz include free-flooded piezoelectric ceramic rings, electromagnetic and hydraulic drivers, tonpilz or piston transducers, and flextensional devices. However, because of their large size and weight, these technologies are not easily adaptable for mounting on advanced smaller underwater vehicle platforms.
There are also two other potential low frequency acoustic source candidates: 1-3 type piezocomposites and cymbal-based flat panels. Present state-of-the-art 1-3 piezocomposites have a thickness of 25.4 mm and although this meets the dimensional requirements, it also means that their acoustic source level at frequencies below 10 kHz is lower than desired. To form thicker 1-3 materials requires extensive electronic matching difficulties and impractical manufacturing and handling requirements. U.S. Pat. No. 6,438,242 to Howarth discloses a cymbal-based flat panel projector that meets the dimensional requirements. In this projector design, miniature flextensional electromechanical drivers that are known as xe2x80x98cymbalsxe2x80x99 are used to drive a stiff radiating plate. In order to realize optimal acoustic output at low frequencies, an air gap between the radiating plates is required. The typical resonance frequencies for the thin panel projectors is less than 2 kHz. The flat panel design does not allow independent addressing of the projectors. Furthermore, the flat panel imposes an averaging affect on the signal received by each projector.
Accordingly, an object of the invention is to reduce the cost of active electro-acoustic transducers by use of inherently inexpensive cymbal-type actuators.
Another object is to do the foregoing with a transducer that is inherently rugged.
Yet another object is to provide an acoustic projector that is small, lightweight, and has low vehicle volume occupation.
Still another object is to provide an acoustic projector that allows independent addressing of each projector element.
Accordingly, the invention provides a compound electroacoustic transducer for producing acoustic signals which has a plurality of elements. Each element has a piezoelectric disk with electrically conductive plates fixed on the top and bottom sides of the piezoelectric disk. A stud is joined to an outer face of each plate. Conductors can be joined to each stud. The elements can be assembled on a resilient structure to form an array. Elements can be used in the array or individually accessed.